Valentin said:
Hi,
3 computers in a home network setup, connected to a Lynksys router.
The router specs show in DHCP Active IP Table:
Computers A, B, C ending in 101, 102, 103 respectively, but on a recent
visit to a financial site, their sever informed me that my IP adress is
competely different.
I would like to find out the "real" IP of each one of my computers, if
possible.
Thanks.
192.168.1.101
|------------------ PC #1
67.189.217.188 |
Internet ---------------- Modem/router ---| 192.168.1.102
|------------------ PC #1
|
| 192.168.1.103
|------------------ PC #1
The local IP addresses used by the computers, as typically
supplied by your router running DHCP, are in the non-routable
192.168.x.x range. They are private addresses, suitable for local
area network (LAN) usage.
The modem/router has one "public" address. The router translates
the private addresses, to the single public address. So in
effect, the three computers are "sharing" one public address.
The reason we use this scheme, is because it allows the limited
supply of public addresses, to be used for communications with
more computers.
The router function that does the translation is called "NAT" or
Network Address Translation.
http://en.wikipedia.org/wiki/Network_address_translation
One of the disadvantages of "NAT", is that if you want to
run an FTP server on one of your three local computers, you need
"port transparency". That means, if someone on the Internet
attempts to reach 67.189.217.188 port 21, you need to make
a special setting in the router, so the packet is "forwarded"
to the correct one of three computers. The special setting means
in fact, that you can only run an FTP server on *one* of your
local computers, if you wish to allow people to FTP stuff
from you. So when setting up various kinds of servers, the
router is nothing but a pain, due to NAT.
http://en.wikipedia.org/wiki/Port_numbers
As an example of things that cause pain, if you wanted to connect
to a Unix box at work, and execute a copy of X-Windows on your PC,
X-Windows display programs are classified as "servers". Which means
you have to set up the appropriate mapping in the router, and then
only *one* PC can be using X-Windows display with respect to a
Unix computer on the Internet. At least, without a lot of
fooling around, and changing port numbers. (And that suggestion is
a dumb one anyway, as you don't really want to leave ports "open"
like that. There is software for making an encrypted connection
between home and work, which reduces the risk of opening ports
on the router.)
When packets come from your router, the address will always be the
same fixed value, like 67.189.217.188. No matter which of the three
PCs the packet came from, it will have the same (public) address.
Your bank (or the USENET server at your ISP), can see the public
address of the packet, and that is what they would report. If,
on the other hand, some software locally, reads your private
address and sends a packet with the information to someone, then
that software could report that (useless) info about your local address.
Generally, you can go into your router's control interface, and
read the public address in there. When I manually log into my
router, and ask it to connect to the Internet via PPPOE, the
public address fetched from my ISP is stored in the router
control panel. And that is how I figure out what my public
address would be.
AFAIK, "ipconfig /all" should be reporting the private address
used on the local LAN.
(Using nslookup in a DOS command window, I can see that
67.189.217.188 maps to comcast.net. I got your public address by
looking at the header of your USENET posting. So, depending on how you
post to USENET, some methods will display your public address.
Other methods are a little less obvious, and use X-Trace to
track where the posting came from -- and in that case, only
an Abuse email to the NNTP server operator, would make anyone
interested in the public IP address. Every time I log off my modem
and log on, I get a new public address from a pool, so my public
IP address is assigned by DHCP, and is never the same twice in
a row.)
Paul
